a) Field of the Invention
The present invention relates to medium carbon steels and low alloy steels and methods of making same. More specifically, the present invention relates to medium carbon steels and low alloy steels having a concentration of tin and/or antimony at the ferrite grain boundaries of the steel which enhances the machinability of the steel. The present invention also relates to processes for producing such steels.
b) Description of the Related Art
"Medium carbon steel" is a term used in the steelmaking art to refer to grades of steel that have carbon contents in the range of about 0.2 to about 0.4 weight percent. Likewise, the term "low alloy steel" refers to grades of steel having a carbon content in the range of about 0.1 to about 0.2 weight carbon and have alloy contents in the range of about 1to about 4 weight percent. These steels are often used in forgings, gears, and other parts for automotive and structural applications. Many of the parts made from these steels are machined at least once during their manufacture. Thus, the machinability of these steels is of great industrial importance.
Machinability, however, is a complex and not fully understood property. A full understanding of machinability would require taking into account a multitude of factors, including the effect of the steel composition, the elastic strain, plastic flow, and fracture mechanics of the metal workpiece, and the cutting dynamics that occur when steel is machined by cutting tools in such operations as turning, forming, milling, drilling, reaming, boring, shaving, and threading. Due to the complexities of the cutting process and the inherent difficulties in making real time observations at a microscopic level, knowledge of the extent of the range of mechanisms that affect machinability is also incomplete.
Lead is used to enhance the machinability of some grades of steel. However, the use of lead has serious drawbacks. Lead and lead oxides are hazardous. Caution must be taken during steelmaking and any other processing steps involving high temperatures. Such process steps produce lead and/or lead oxide fumes. Atmosphere control procedures must be incorporated into high temperature processing of lead-bearing steels. Disposal of the machining chips from lead-bearing free-machining steels is also problematic due to the lead content of the chips. Another serious disadvantage is that lead is not uniformly distributed throughout conventional steel products. This is because lead is not soluble in the steel and, due to its high density, it settles out during the teeming and solidification processes, resulting in segregation or non-uniform distribution within the steel. Furthermore, lead can envelope manganese sulfide inclusions in the steel thereby decreasing the steel's mechanical properties and service life through reduced resistance to impact and fatigue stresses.
Steels have also been developed in which machinability is enhanced through the use of inclusions in the steel. Manganese sulfide inclusions are often employed to promote machinability. Sometimes a soft phase, such as a low melting metal like bismuth or a plastic oxide, such as a complex oxide containing calcium, selenium or tellurium, is used to surround manganese sulfide inclusions to further enhance machinability. However, strategies that improve machinability through the use of inclusions have the disadvantage that the inclusions remain in the steel's microstructure after machining. The presence of inclusions in the steel may lower the steel's mechanical properties and lower its service life through reduced resistance to impact and fatigue stresses. Thus, although inclusions may be used as agents to improve machinability in some grades of steel, a price is paid in terms of reduced mechanical properties and service life.